Devices for digitizing documents, by converting images on the document into digital data form for subsequent use in reprinting a copy of the original, have heretofore been known. In such systems, a light source is typically directed toward the document while an array of photo detectors scans the document. During scanning, the array detects light reflected from the images on the document. Digital data is generated corresponding to the detected light, and thus to the images. In such devices, it is important to identify the location on the document from which data is being generated relative to a reference location. Unless there is a one-to-one correspondence between locations of a print from which data is being digitized and corresponding locations of a copy of the print subsequently reproduced from the data, the copy does not duplicate the original.
In one prior art apparatus, original prints are mounted to a rotatable drum and then optically scanned by a scanner as the drum rotates. As the drum is rotated, the optical scanner is linearly shifted in a direction parallel to the axis of the drum. This direction corresponds to the X-coordinate axis of a two-dimensional cartesian coordinate system. In order to accurately monitor the position of the scanner along the X-axis, relatively expensive feed-back systems, such as laser feedback systems, have been employed.
Although not known to be utilized in an optical digitizing apparatus, one device used in monitoring positions along a single axis comprises a digital readout apparatus for a milling machine produced by the Sony Corporation and designated the Magnascale LF-200. This apparatus utilizes a metal bar which is magnetically coded along its length with a position indicating code. A sensor is shifted along the bar and reads the code. From these readings, the position of the sensor along the bar, relative to a reference position, is known.
To establish a Y-coordinate axis for digitization of a document, one prior art approach employs an expensive mounting mechanism for the optical scanner. This mounting mechanism guides the linear movement of the scanner along a line which precisely parallels the drum axis. Then, with the scanner at a given linear position and as the drum rotates, the scanner scans the print in a Y-coordinate axis direction. However, if the scanner mounting mechanism does not move the scanner along a line which is virtually exactly parallel to the drum axis, the data is incorrectly shifted in the Y-axis direction relative to data from adjacent scans of the document. As a result, reproduced copies of the original have distorted images.
Therefore, a need exists for a relatively inexpensive apparatus which reliably calibrates an optical digitizer, and which establishes an orthogonal reference coordinate system for the digitizer, without the need for costly scanner mounting mechanisms.